## What’s the difference between PSI and CFM?

The difference between PSI and CFM are what they measure. PSI measures pressure, while CFM measures volume. PSI and CFM are often used as performance specifications for air compressors. Together, they indicate the maximum air volume and pressure produced by an air compressor to power air tools. To better understand the difference between PSI and CFM, let’s learn what PSI means and what CFM means.

## What Does PSI Mean?

The initials PSI stand for Pounds per Square Inch. PSI measures how many pounds of pressure (force), is in an area, specifically in one square inch. The force of the air is what gives compressed air its power. For example, an air compressor’s output could be rated for 100 psi*, which means that 100 pounds of pressure is delivered per square inch.

*PSI is typically written in lower case letters, psi, in air compressor specs.

## What does CFM mean?

CFM means Cubic Feet per Minute. CFM measures the volume of air in cubic feet for each minute it moves. In the case of an air compressor, CFM indicates how much air can move per minute. For example, an air compressor’s output could be rated for 30 CFM, which means 30 cubic feet of air is flowing per minute.

### How Do CFM and psi Relate?

CFM and psi relate to each other in an important way that ensures the proper operation of an air compressor. For a tool to operate and perform optimally, both CFM and psi must be sufficient.

Let’s look at a real-life example to help understand how CFM and psi relate:

Imagine you have a garden hose, and you turn it on. Water will flow out, and it might reach a few feet past the end of the hose, perfect for filling up your bucket or watering can. If you take the garden hose and restrict the space at the end of the hose with your thumb to create less room for the water to flow out the water will shoot out with much more pressure than before. Even though the hose is producing the same amount of water, the extra pressure will allow the water to travel faster and further, perfect for having a water fight!

Hopefully this hose example is relatable, and even though the example used water to describe the relationship between CFM and psi, the concept is the same with air.

Consider this second real-life air example. There’s a tunnel with wind blowing through it, and the tunnel gets smaller and smaller. As the wind blows into the tunnel, it travels through the shrinking space, and starts to feel stronger and stronger. This is because even though the volume of air (CFM) remains the same throughout the tunnel, the air is being squeezed into a tighter space, resulting in the pressure (psi) increasing.

In both examples we shared how psi relates to CFM. While having a sufficient volume of air to power air tools is important, it’s also crucial to ensure there’s enough psi (pressure) to give the air power.

#### What Size Air Compressor Do I Need To Run My Air Tools?

Now that you understand the difference between psi and CFM, you might be wondering how to be sure you have the right air compressor to run your air tools. The easiest way to do this is to check the air requirements of your air tools (both CFM and psi) and then ensure the air compressor you choose meets those requirements.

But it’s more than simply matching the CFM and psi of your tools to your air compressor. It also matters whether you have an air receiver tank, a rotary screw air compressor or reciprocating air compressor, and even what your application is.

To continue your research into psi and CFM, check out these helpful articles:

## How To Calculate Vessel Depressurization Time With Quick & Simple Logic

VMAC recently received a message from a process engineer in London who had a great question after reading our How to Work Out ‘Time To Fill’ Type Questions Using Simple Logic article. The process engineer asked:

“I was wondering if you have a similar calculation that you could share for “How to Calculate the Depressurization Time”? For instance, I have a pipe section at a certain pressure and I depressurize it to certain lower pressure through a 1″ hole (via a 1″ valve). How long would it take the pipe section to reduce pressure from P1 to P2?”

## What are Dual Tower Regenerative Desiccant Air Dryers (and how do they work?)

#### Heaterless Type (Pressure Swing Dryers)

Dual tower desiccant air dryers are used to produce dewpoint temperatures below the freezing point of water, as well as reduce the moisture content of compressed air used in critical process applications. Typical dewpoints produced by these types of dryers are -40° F to -100° F, although lower dewpoints are possible. Continue reading “What are Dual Tower Regenerative Desiccant Air Dryers (and how do they work?)”

## Gas combustion engines at elevation

Small CFM gas driven air compressors won’t perform as well at 5,000 feet of elevation as they do at sea level. They may fail, bog down, or supply less air power. But why is this?
It all comes down to the physics of gas combustion engines.
Continue reading “Gas combustion engines at elevation”

## ACFM (Actual Cubic Feet per Minute) vs. SCFM (Standard Cubic Feet per Minute)

The air compressor industry loves to use acronyms, especially when measuring airflow and air consumption. Two common acronyms, representing measurements, are ACFM and SCFM.

CFM—or Cubic Feet per Minute—is an important metrics when choosing an air compressor and related pneumatic equipment. Pneumatic tools and equipment require a specific minimum air mass, or CFM, to perform properly on the job site.

But what exactly does it mean when manufacturers use ACFM or SCFM to perform properly on the job site?

## What is ACFM?

ACFM (Actual Cubic Feet per Minute) may have different definitions depending on the industry.  VMAC defines ACFM as the true air mass flow given a certain set of real-life conditions. We will simulate a real life environment and then measure the CFM based on the actual output of air, resulting in ACFM.

But ACFM is impacted by atmospheric conditions and the surrounding environments. For example, an air compressor on a mountain top is likely to have lower output than that same air compressor will have at ocean level.

## What is SCFM?

SCFM (Standard Cubic Feet per Minute) measures air output, like ACFM, but uses a standard that takes atmospheric conditions into account.

SCFM is a set of specific parameters determined by the American Society of Mechanical Engineers (ASME) that are recognized across many industries. These SCFM calculations are based on atmospheric conditions (“standard conditions”) to measure air mass flow from an air compressor.

SCFM “standard conditions” include:

• atmospheric pressure at sea-level of 14.7 PSIA (760 mmHg),
• relative humidity of 36%, and
• ambient temperature of 68°F (19°C).

After completing the calculations using these conditions, the maximum SCFM output of the air compressor is revealed.

SCFM is the only way to compare “apples to apples”, or air compressor to air compressor, as operating conditions may otherwise vary depending on where the air compressor air mass flow is being measured. VMAC defines SCFM as the air mass flow generated at “standard conditions”.

## Differences Between ACFM & SCFM

At “standard conditions”, with no efficiency losses, SCFM equals ACFM.  Where inlet conditions vary from “standard conditions”, consideration may be necessary to ensure the specified air compressor has enough power to generate adequate air mass flow for the tool to perform properly on the job site.

ACFM demand by a tool may be higher if any one of the following conditions is different from standard conditions:

• atmospheric pressure is lower (elevation)
• humidity is higher
• temperature is higher

Another way of thinking about this is the air compressor needs to work harder as the job site drifts away from “standard conditions”.  A more powerful air compressor (with higher SCFM rating) may be required to generate adequate ACFM for the pneumatic tool to perform properly on the job site.

Quick Calculations:

• For every 1000 ft (305 m) of elevation increase over “standard conditions”, ACFM demand increases by approximately 5%.
• For every 20oF (11.1oC) of ambient temperature increase over “standard conditions”, ACFM demand increases by approximately 5%.
• For every 20% of humidity increase over “standard conditions”, ACFM demand increases by approximately .5%.

Example Calculation:

A 1” impact gun may have a minimum air mass flow requirement of 45 CFM.  On the job site in Ft McMurray, Alberta at an elevation of 1,211 ft (369 m) on June 22, 2011 at 2:00PM, atmospheric pressure is 14.2 PSIA (732 mmHg), humidity is 24%, and temperature is 84oF (27oC).   For an air compressor to generate 45 ACFM in these conditions, it actually needs to have an SCFM rating of approximately 49 SCFM.

## Why Does ACFM vs. SCFM Matter?

You need to ensure your air compressor system produces enough air to properly power your tools. If the environment you work in impacts air production, SCFM ratings may not perfectly reflect your real-world needs. Unless you are only using your air compressor on the beach on a cool spring day, add a buffer when specifying your equipment.

## Most Common Compressed Air Drying Methods

As mentioned in previous articles, when compressing air, liquid is also brought into the air stream.  When cooling, that liquid condenses and is delivered with the compressed air to your tool or application.  There are a number of ways of removing or reducing the amount of liquid in the air stream.  These include:

• Storage Tank Cooling Method
• After-cooling
• Air-cooled versions
• Water Cooled Versions
• Absorption Drying

## Keeping Your Cool: Managing the heat rejection requirements of your air compressor system

If you’ve determined what size of compressor you need and the engine requirements to power that compressor the next important step is to ensure that the combination is going to operate optimally in your working environment.  In the case of mobile compressors this may include high altitudes, dirty environments and large ambient air temperature fluctuations.  For this article, we will focus on the different methods of rejecting the heat generated by your compressor system. Continue reading “Keeping Your Cool: Managing the heat rejection requirements of your air compressor system”

## How inlet temperature affects air flow

It is important to consider where your compressor air intake is located. Some factors to consider are: particulates in the air (dust which can plug filters), ingestion potential (can the intake become plugged with snow or mud) and the temperature of the air when it enters the compressor.  If you are mounting an air compressor to an internal combustion engine and/or in any type of enclosure the ambient air temperature around the compressor can rise well above the outside ambient temperature causing a reduction in compressor performance. Continue reading “How inlet temperature affects air flow”

## How to work out “Time to Fill” type questions using simple logic

Say you have an air tank or some other suitably pressure rated vessel and you want to figure out how long it will take to bring the pressure up to a certain level using a compressor of known capacity (CFM) and you want to do it using common sense, not a lot of formulas.

## Why does my compressor system perform differently at higher altitudes?

Anyone who has had to use a compressor at a high altitude knows how frustrating it can be. Compressor performance is degraded and it can take a lot longer to complete tasks. People who move to or work in areas that are at a higher altitude are often surprised when their compressor seems to be working slower than normal.

#### How an Air Compressor System Works

Mobile air compressor systems commonly in use typically consist of a gas or diesel engine powering an air compressor. When you turn on an air compressor system, it draws in ambient air through the compressor intake and compresses it to a smaller volume. This compressed air can then be stored in a storage tank or used to directly power your tools and equipment. Once the air storage is full or the tools being used are no longer running, the compressor stops drawing in and compressing air by either shutting off the engine and compressor or entering some sort of a standby mode.
Continue reading “Why does my compressor system perform differently at higher altitudes?”